1. Field of the Invention
This invention relates to an engine control system for a hybrid vehicle and, more particularly, to an engine control system for a hybrid vehicle which can maintain stopping the engine and deceleration fuel supply cut by determining the driver intention to stop the engine based on the manipulation of the clutch and the engine speed at the time of the clutch manipulation.
This application is based on Japanese Patent Application No. 11-62413, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In series hybrid vehicles, the engine drives a generator whose electric power output is used to drive the motor which, in turn, drives the wheels. In parallel hybrid vehicles, the motor coupled to the engine assists the rotation of the drive shaft of the engine while charging a battery with electric energy using the motor itself as a generator or using a separate generator.
Because the engines of such hybrid vehicles can constantly run within an engine-speed range suitable for producing high fuel mileage and a low emissions level or their drive loads can be reduced, the hybrid vehicles can achieve lower fuel consumption and lower emissions levels than can conventional engine vehicles.
As disclosed in, for example, Japanese Unexamined Patent Application, First Publication Hei 8-317505, some of those hybrid vehicles are designed to be capable of stopping the operation of the engine in accordance with predetermined driving conditions.
Because of the ability to stop the operation of the engine, such a hybrid vehicle is superb in preventing over-charging of the battery or achieving a further improvement in fuel consumption but has the following problem.
When the fuel supply is cut during deceleration, and when the driver tries to accelerate the vehicle by manipulating the clutch and changing the gear, some shocks or engine stalling may occur at a low engine speed (this problem does not occur when the engine speed is at a certain speed or higher) at the time of restarting the engine following the engagement of the clutch. This may give the driver an uncomfortable feeling.
There is, therefore, a demand for a means which can ensure smooth driving that reflects the driver intention by determining from various manipulations done by the driver including the aforementioned clutch manipulation if the driver intends to stop the vehicle.
Accordingly, it is an object of the present invention to provide an engine control system for a hybrid vehicle, which determines if the driver intends to stop the vehicle when the clutch is disengaged, and can therefore smoothly stop the engine.
To achieve this object, according to one aspect of this invention, an engine control system for a hybrid vehicle, having an internal combustion engine (e.g., an engine E in one embodiment) and an electric motor (e.g., a motor M in the embodiment) as driving force sources, comprises a clutch, provided between the engine and a transmission system (e.g., a transmission system T in the embodiment), for disabling and enabling driving force transmission between the engine and the transmission system; clutch disengagement detector (e.g., a clutch switch in the embodiment) for detecting engagement and disengagement of the clutch; engine speed detector (e.g., an engine speed sensor S5 in the embodiment) for detecting an engine speed (e.g., an engine speed NE in the embodiment); and fuel cutter (e.g., a process by an engine ECU 1 in the embodiment) for permitting stopping and restarting of the engine in accordance with at least a result of detection by the clutch disengagement detector among predetermined drive conditions, and cutting fuel supply to the engine when the vehicle decelerates, a fuel-supply restart engine speed (e.g., an engine speed of 1000 to 1500 rpm in the embodiment) at which fuel supply is resumed being set in the fuel cutter, wherein stopping the engine is accomplished by fuel supply cut by the fuel cutter, and once the engine stop control is initiated, when the engine speed reaches the fuel-supply restart engine speed, fuel supply cut by the fuel cutter is maintained, and when the engine speed detected by the engine speed detector at a time disengagement of the clutch is detected by the clutch disengagement detector (e.g., a low engine speed NECLSW when the clutch is actuated in the embodiment) is equal to or lower than a predetermined engine speed (e.g., a low engine-speed upper limit NECLMG when the clutch is actuated in the embodiment), cutoff of fuel supply by the fuel cutter is maintained whereas when the detected engine speed is higher than the predetermined engine speed, fuel supply is restarted.
With the above structure, when the engine speed with the clutch disengaged is equal to or lower than a predetermined engine speed equal to or lower than the idling speed, fuel supply cut is maintained considering that the driver has an intention to stop the vehicle even if the engine speed has reached the fuel-supply restart engine speed set in the fuel cutter. This can guarantee engine control that fulfills the driver""s intention to stop the vehicle and reflects the driver""s intention.
In this engine control system, fuel supply cut by the fuel cutter may be maintained at least when the shift position of the vehicle running at a vehicle speed equal to or lower than a predetermined value (e.g., a vehicle speed upper limit VFCMGST at which fuel cutting during deceleration is maintained in step S123 in the embodiment) is other than a reverse position (reverse) and low position (first gear). For example, the condition involves a decision on the xe2x80x9cshift positionxe2x80x9d in step S110 as to whether the shift position is the reverse position or another position and a decision in step S125 on the state of a flag F_NGRMG for determining if the shift position during deceleration is other than the low position (first gear) in the embodiment. This structure can ensure engine control that reflects the driver""s intention more adequately.
In the engine control system, a condition for stopping the engine is that the brake pedal is actuated (e.g., xe2x80x9cONxe2x80x9d as the result of the decision in step S128 in the embodiment). This structure can allow the engine to be stopped according to the driver""s intention to stop the vehicle.
The predetermined engine speed (e.g., a low engine-speed upper limit NECLMG when the clutch is actuated) is equal to or lower than an idling speed.